High-powered DC voltage electrical systems are undergoing major development. Indeed, many transportation systems include a DC voltage power supply.
Combustion/electric hybrid vehicles or electric vehicles have high-powered batteries. Such batteries are used to drive an AC electric motor by means of an inverter. The voltage levels needed for such motors can be hundreds of volts, typically on the order of 400 Volts. Such batteries generally also have high capacity in order to favor the autonomy of the vehicle in electrical mode.
To obtain high values of power and capacity, several groups of accumulators are placed in series, often called “stages.” The number of stages and the number of accumulators connected in parallel in each stage vary according to the voltage, the current and the capacity desired for the battery. The association of several accumulators is called a battery of accumulators.
The electrochemical accumulators used for such vehicles are generally lithium-ion type accumulators because of their capacity to store substantial energy with a limited weight and volume. Lithium-ion iron phosphate (LiFePO4) type batteries are undergoing major development because of a high intrinsic level of security, to the detriment of energy storage density which is somewhat lagging. An electrochemical accumulator usually has a nominal voltage of around 3.3 V for lithium-ion iron phosphate LiFePO4 technology, and around 4.2 V for cobalt-oxide based lithium-ion type technology.
The charging or discharging of an accumulator respectively results in an increase or decrease in the voltage at its terminals. An accumulator is considered to be charged or discharged when it has reached a voltage level defined by its electrochemical process. In a circuit using several stages of accumulators, the current flowing through the stages is the same. The level of charging or discharging of the stages therefore depends on the intrinsic characteristics of the accumulators. Differences in voltage between the stages appear during the charging or discharging owing to differences in manufacturing, aging, assembling and operating temperature between the different accumulators.
For an accumulator based on Li-ion technology, an excessively high or excessively low voltage, known as a threshold voltage, can damage or destroy the accumulator. For example, the overcharging of a cobalt-oxide-based Li-ion accumulator can lead to its thermal stalling and an outbreak of fire. For an iron-phosphate-based Li-ion accumulator, overcharging can lead to a decomposition of the electrolyte, which diminishes its service life or impairs it. An excessively great discharge, which leads to voltage below 2 V for example, causes mainly an oxidation of the current collector of the negative electrode when it is made of copper and therefore a deterioration of the accumulator. Monitoring the voltages at the terminals of each stage of accumulator(s) is therefore obligatory during the charging and discharging for reasons of security and reliability. A monitoring device is thus generally placed in parallel on each stage and enables this function to be fulfilled.
The function of the monitoring device is to track the state of charge (or residual charge) and discharge of each stage of accumulators and to transmit information to the control circuit in order to stop the charging or discharging of the battery when a stage has reached its threshold voltage. However, in a battery with several series-connected stages of accumulator(s), if the charging is stopped when the stage most charged reaches its threshold voltage, then it is quite possible the other stages have not yet been totally charged. Conversely, if the discharging is stopped when the stage most discharged has reached its threshold voltage, then it is quite possible that the other stages will not yet have totally discharged. In this case then, the capacity of each stage of accumulators is not exploited. This represents a major problem in transportation type applications with embedded batteries having high constraints of autonomy. To cope with this problem, the monitoring device is generally associated with a charge-balancing device.
The balancing device has the function of optimizing the charge of the battery, and therefore its autonomy, in leading the stages of series-connected accumulator(s) to a state-of-charge and/or state-of-discharge that is identical. There are two categories of balancing devices: energy dissipation devices and energy transfer devices.
In energy dissipation balancing systems, the voltage at the terminals of the stages is made uniform by diverting the charging current from one or more stages that have reached the threshold voltage and by dissipating the energy in a resistor. Alternatively, the voltage at the terminals of the stages is made uniform by discharging one of the stages that has reached the high voltage threshold.
However, such energy dissipation balancing systems have the major drawback of consuming more energy than necessary to charge the battery. Indeed, it is necessary to discharge several accumulators or divert the charging current of several accumulators so that the last accumulator or accumulators that are slightly less charged can complete their charging. The dissipated energy can therefore be much higher than the energy of the level or levels of charging still to be performed. Furthermore, they dissipate excess energy in the form of heat. This is not compatible with the constraints of integration into transportation type embedded applications, and significaly reduces service life of the accumulators when the temperature rises.
The energy transfer balancing systems exchange energy between the auxiliary battery or an auxiliary energy network and the stages of accumulators.
The energy transfer can be either one-directional, from the battery to the stages or the stages to the battery, or bidirectional, from the battery to the stages and from the stages to the battery or from adjacent stage to adjacent stage.
To limit losses during energy transfers, the patent application, FR11/51724, which has not been published as the priority date of the present application, describes an improved balancing device. This document proposes the use of a structure based on the principle of a flyback converter to ensure balancing of the stages of accumulator(s) through the discharging of the stage or stages for which the state or states of charging and/or the capacity or capacities are higher as well as energy transfer from the battery of accumulators, known as power batteries, to the auxiliary network of the vehicle. The use of a buck converter needed to power the low-voltage auxiliary network from a high voltage battery can thus be avoided. Furthermore, the auxiliary network can be powered solely by balancing devices without the use of an auxiliary battery.
To simplify the regulation of the parallel-connected balancing devices, the devices are controlled individually. Thus, the accumulator(s) stage that has the highest capacity and/or state-of-charge supplies the energy to the auxiliary network. The individual operation of the balancing devices requires the active balancing device to provide all the energy required on the auxiliary network. The sizing of the balancing devices must therefore be done accordingly.
To reduce and distribute the power provided by each balancing device, it is known, for example from U.S. Pat. No. 4,717,833, to simultaneously use different balancing devices. In particular, the '833 patent proposes a command using a method of regulation providing for the sharing of the currents between the balancing devices so as to prevent certain converters from having to work more than others because of disparities between these converters. The '833 patent presents a method of interdependent control of parallel-connected converters. The method of control implements a measurement of current and a measurement of voltage by and on each converter and makes use of an interconnection bus between the converters. The interconnection bus enables the exchange of information on the sharing of the currents between the converters.
Because of this, the regulation loops are dependent on one another and have to be synchronized. This physical link between converters must be insensitive to parasitic noises and must in no case cause the converters to stop working when it is in an open state or in a short circuit. This physical link must furthermore be sized according to the presence of the different levels of voltage between the stages or elements, with amplitudes. Implementation of this scheme is complex and costly in its development and in its manufacture.